A Programmable Logic Controller (PLC) is an industrial computer used to control and automate complex systems. PLCs are typically found in factory type settings. PLCs are used to control robots, assembly lines and various other applications that require a large amount of data monitoring and control. In addition to the PLC itself, there are many different components that can be connected to the PLC using either an open protocol, such as, for example, RS-232, or a proprietary protocol, such as, for example, Allen-Bradley® Remote I/O (input/output) available from Rockwell Automation, Milwaukee, Wis. The PLC and all of the peripheral components linked to the PLC define the control system.
PLCs can be programmed using different development platforms. One common development platform is called ladder logic. Ladder logic is a programming language based on executing commands on a line-by-line permission system. As a PLC program executes, the processor checks to see if there is a permissive statement on the left side of a ladder rung. If that permissive statement is true or if there is no permissive statement, the processor performs the operation on the right side of the ladder rung. When one rung is executed, the processor moves to the next rung down of the ladder in a sequential fashion. This process is called a ladder scan. Depending upon the manufacturer of the PLC, the exact scan sequence may be different. Typically, the PLC will determine if the states of the various peripherals, such as a Remote I/O, have changed since the previous scan. If the state of any peripheral has changed, the PLC will update the information in its memory, and then perform its ladder scan based upon the most current information.